No link between polymetallic nodules and anomalous oxygen concentration increases in benthic experiments from the...

Polymetallic nodules in the deep sea of the Clarion Clipperton zone (CCZ) have been implicated in the abiotic production of oxygen during benthic chamber respirometer experiments. The hypothesis, termed "dark oxygen", was supported with a correlation between average nodule surface area and oxygen production rates, given as a Spearman's correlation coefficient of ⍴ = 0.664, p = 0.031, n = 11, suggesting that polymetallic nodules were responsible for the oxygen concentration rises and that larger polymetallic nodules cause more oxygen production. However, this correlation and the dark oxygen hypothesis appears to be incompatible with the observation that experiments in the western CCZ and a control experiment recorded increasing oxygen with no nodules present.

To investigate the dark oxygen hypothesis, we repeated the surface area analysis using image segmentation, and expanded the dataset with three additional chamber images, the weight of the nodules recovered from the chambers and a further three experiments for which nodule weight data are available.

We find no significant correlations between total nodule area, average area, total nodule weight or average nodule weight and net oxygen concentrations, increase in the chambers. The inclusion of the three additional chamber images in the analysis reduces the significance of the previously stated correlation to ⍴ = 0.433, p = 0.122, with confidence intervals that cross zero (-0.13, 0.78).

On further scrutiny the correlation between oxygen production and nodule surface area is spurious, and consequently, there is no established causal link between polymetallic nodules and increasing oxygen concentration in the benthic chamber respirometer experiments. Since oxygen concentration increased in experiments regardless of the size, abundance, or presence of polymetallic nodules, we find it likely that polymetallic nodules are not related to the rising oxygen concentration in any of these experiments.